Recently, with continued demand for improvement of fuel efficiency of vehicles and stringent regulations on emissions from vehicles in many countries, demand for eco-friendly vehicles has increased. As a practical representative thereof, hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) have been developed.
A hybrid vehicle runs using two power sources including an engine and a motor. For a harmonious operation of the engine and the motor, optimal output and torque may be generated. Specifically, in the case of a hybrid vehicle equipped with a parallel-type or TMED-type (Transmission-Mounted-Electric-Device-type) hybrid system, in which an electric motor and an engine clutch (EC) are installed between an engine and a transmission, an output of the engine and an output of the motor may be transmitted to a drive shaft simultaneously.
Typically, in an initial stage of acceleration, the hybrid vehicle runs using electrical energy (i.e. an EV mode). However, there is a limitation in providing enough power requested by a driver using only electrical energy, and there is thus a need to use an engine as a main power source (i.e. an HEV mode) at certain moments. At such times, when the difference between the number of revolutions per minute of the motor and the number of revolutions per minute of the engine is within a predetermined range, an engine clutch is engaged such that the hybrid vehicle operates as the motor and the engine rotate together.
However, the engine may not be used as the main power source even in the HEV mode. For example, in a parallel mode of the HEV mode, the power of the engine functions as driving force. On the other hand, in a series HEV mode, the engine is driven with low load and thus the driving force of the engine is used to generate electricity. This will be described with reference to FIG. 1.
FIG. 1 is an illustrative view showing change to a series HEV mode for generation of electricity in a general hybrid vehicle.
Referring to FIG. 1, while the hybrid vehicle is driven in an EV mode, when a state-of-charge (SOC) value becomes equal to or less than a predetermined value (discharge limit), an engine operates to perform charging with power of the engine. At this time, when the vehicle is stopping, since the power of the engine is not used for driving and the operating point of the engine is low, non-driving fuel loss may further increase.
Problems occurring due to such an engine operating point will be described with reference to FIG. 2.
FIG. 2 is an illustrative view showing a relationship between an operating point and efficiency according to the HEV mode of a general hybrid vehicle.
FIG. 2 shows a brake specific fuel consumption graph (BSFC) representing engine efficiency, wherein a horizontal axis of the graph indicates an engine RPM and a vertical axis of the graph indicates a vehicle speed. Efficiency gradually increases toward the center of a circular ISO efficiency curve.
As shown in the figure, a parallel driving region 10 is set in a relatively high efficiency region. In contrast, a series engine driving region 20 is generally set in a low RPM region 1100 to 1300 in consideration of vibration, noise and the output of a motor (e.g., hybrid starter generator (HSG)) which will generate electricity.
Meanwhile, in recently released vehicles, a full automatic temperature controller (FATC) is responsible for performing temperature control. In hybrid vehicles, as necessary, the FATC performs control to heat indoor air using engine coolant heated by heat of the engine. More specifically, when a positive temperature coefficient (PTC) heater is not mounted in a hybrid vehicle or when a PTC heater having low capacity is mounted, the FATC may determine that engine coolant is utilized. At this time, the FATC requests operation of the engine from a hybrid control unit (HCU), when the temperature of engine coolant is less than that of water necessary for heating.
Then, the HCU operates the engine and selects any one of the parallel mode and the series mode depending on situations. However, as described above with reference to FIG. 2, since the parallel mode is better than the series mode in terms of engine efficiency, when the parallel mode is possible (when a vehicle runs at a predetermined speed or more), the parallel mode may be preferentially selected.
However, when a short stop occurs in the parallel mode and the driving mode is changed to the series mode based on the operating point of the engine during such a short stop time, an inefficient control result is obtained. This will be described with reference to FIG. 3.
FIG. 3 is an illustrative view showing problems occurring due to HEV mode change control based on a vehicle speed in a general hybrid vehicle.
FIG. 3 shows two graphs: an upper graph and a lower graph. Horizontal axes of the two graphs indicate time, a vertical axis of the upper graph indicates vehicle speed, and a vertical axis of the lower graph indicates driving mode of a hybrid powertrain. That is, in the lower graph, a lowest point of the vertical axis indicates an EV mode, a middle point thereof indicates an HEV mode and a highest point thereof indicates a parallel HEV mode.
Referring to FIG. 3, as the vehicle speed reaches to a value enabling to enter the parallel mode after the vehicle departs from a state of stopping in the series mode, the driving mode of the vehicle may be changed to the parallel mode. Even when the vehicle stops for a relatively short period of time due to, for example, traffic signals while being driven in the parallel mode, if the vehicle speed enabling to enter the parallel mode is not satisfied due to the stop, the driving mode of the vehicle is temporarily changed to the series mode. As a result, the engine operates at a low-efficiency operating point during such a short period of stop time and coolant temperature increase is insignificant due to low engine output.